The present invention relates to the field of nondestructive testing, and provides an interferometric method and apparatus for measuring deformations in diffusely reflective objects.
It has been known to test objects for defects using interferometric methods such as holography or shearography. In shearography, two laterally displaced (sheared) images of the same object are made to interfere with each other, resulting in a shearographic image. Embedded in the shearographic image is a random interference pattern. When the object is deformed, the interference pattern is slightly changed. By digitally comparing two shearographic images corresponding to deformed and undeformed states of the object, a recognizable fringe pattern is produced. Analysis of the fringe pattern reveals information about the deformation of the object, and, in particular, can indicate the presence or absence of a defect. Shearography is especially useful in testing bonded structures such as those found in aircraft or spacecraft, particularly where a potentially imperfect bond lies well below the surface of the structure.
An early description of the technique of shearography was presented by Hung and Taylor, in xe2x80x9cMeasurement of slopes of structural deflections by speckle-shearing interferometryxe2x80x9d, Experimental Mechanics, vol. 14, pages 281-285 (1974). Shearography was originally practiced by forming the interference pattern on photographic film.
Eventually, it was recognized that nondestructive testing could be performed much more efficiently if it were done electronically, i.e. by replacing the photographic film with a video image sensor (such as a charge coupled device) and comparing the shearographic images digitally, and displaying the fringe pattern on a video monitor. However, the inherent low resolution of video monitors, as compared with photographic film, limited the utility of electronic shearography systems.
A key element enabling the recording of shearographic images on a low resolution image sensor is an image shearing device which can produce an interference pattern of sufficiently low spatial frequency. The first practical apparatus for performing electronic shearography was described in U.S. Pat. No. 4,887,899, the disclosure of which is incorporated by reference herein. In the latter patent, the shearing effect is produced by the action of the double refraction of a birefringent material and a polarizer. Later patents have disclosed other methods of producing sheared images. For example, U.S. Pat. No. 5,094,528 discloses a shearography system resembling a Michelson interferometer. In the latter system, two light beams, taking slightly different optical paths, produce the sheared images which are made to interfere to form a composite interference pattern. The disclosure of the latter patent is also incorporated by reference herein.
The present invention provides a novel system and method of non-destructive testing that has several advantages over those of the prior art. In the present invention, the light collection efficiency is higher, and the degree of shearing is continuously and easily adjustable. The invention allows easy comparison of displacements of two distinct points on an object, or even of points located on different objects. The invention enables all of the above to be accomplished in a manner such that the resulting interference patterns can be recorded on a low-resolution video image sensor. The invention is also capable of introducing phase shifts and carrier fringes for automation of phase determination.
In one embodiment, the system of the present invention comprises a source of coherent light, positioned to illuminate a test object, and an image sensor connected to a computer. The image sensor may be a CCD camera or its equivalent. A pair of mirrors, one of which is partially reflective and the other of which is totally reflective, are positioned to direct light reflected from the test object into the video camera. Due to the partially reflective nature of one of the mirrors, and due to the orientation of the mirrors in a non-parallel arrangement, the system produces two laterally sheared images of the object, one formed of light reflected from the first mirror and the other formed of light reflected from the second mirror. These images are made to interfere with each other to produce a shearographic image which comprises a random interference pattern. By proper adjustment of the angles and/or spacing of the mirrors, the rays defining the two images become collinear, or nearly so, thereby producing an interference pattern having a very low spatial frequency, and hence one that is recordable on a video image sensor.
The invention can be used to compare displacements of two distinct points on a single object, or of two points on two different objects. To analyze points on different objects, the angle formed by the mirrors will usually be greater than in the case where a single object is being analyzed.
Analysis of the interference patterns obtained with the present invention need not be limited to qualitative examination of fringes. Instead, by using appropriate algorithms, one can determine the phase of the light received at every pixel of the image, using easily obtained measurements of light intensities, with and without deformation of the object, taken at known phase shifts. The phase of the light can be easily adjusted by varying the refractive index of a medium disposed between the mirrors, or by moving the mirrors themselves. In another alternative, one can derive phase information by superimposing a xe2x80x9ccarrierxe2x80x9d fringe pattern, and performing Fourier analysis on the resulting interference pattern.
The invention can also be used to detect leaks in sealed packages, insofar as gas leaking from a package changes the density, and hence the refractive index, of the air in the vicinity of the package. The change in refractive index will change the interference pattern obtained with the methods described above, and can indicate a leak.
The present invention therefore has the primary object of providing a method and apparatus for nondestructive testing of diffusely reflective objects.
The invention has the further object of providing a method and apparatus for performing electronic shearography, in nondestructive testing of diffusely reflective objects.
The invention has the further object of providing a method and apparatus as described above, wherein the method can be used successfully with a video camera having low resolution.
The invention has the further object of providing a method and apparatus for determining relative displacements between two distinct points on an object, or between points on two different objects.
The invention has the further object of providing a nondestructive testing apparatus and method having improved efficiency.
The invention has the further object of providing a shearography system and method wherein the amount of shearing can be easily varied.
The invention has the further object of providing a nondestructive testing system having a simplified optical arrangement, and which is therefore robust, compact, and relatively insensitive to vibration and environmental disturbances.
The invention has the further object of providing a nondestructive testing system in which interference patterns formed in a video camera can be readily analyzed automatically.